Synthesis gas consists essentially of carbon monoxide and hydrogen, but may also comprise carbon dioxide.
The component reactions essential for the preparation of synthesis gas from hydrocarbons are shown in the formulae (I to III) below. The formulae relate to the conversion of methane as the hydrocarbon. For homologues of the hydrocarbon methane, correspondingly stoichiometrically corrected formulae apply, but these are likewise common knowledge.CH4+H2OCO+3·H2  (I)CO+H2OCO2+H2  (II)CH4+2·H2OCO2+4·H2  (III)
The reactions according to the formulae (I) and (III) are strongly endothermic and represent the significant reactions in connection with synthesis gas preparation. The reaction according to formula (II) is the reaction formula known to those skilled in the art under the name “water-gas shift reaction” and is exothermic. All three reactions according to the formulae (I to III) are equilibrium-limited.
The synthesis gas obtained from such reactions constitutes an essential starting material for further conversion, for example, to tailored long-chain hydrocarbons by the Fischer-Tropsch process.
The controlled supply of heat in processes for obtaining synthesis gas is important since the position of the equilibria of the aforementioned reactions according to the formulae (I to III) is highly dependent on the temperature of the reaction zone, and hence the yields and/or selectivities for hydrogen and/or carbon monoxide can be controlled as a result.
An uncontrolled temperature decline as a result of the endothermic reactions according to the formulae (I) and/or (III) can thus promote the formation of greater or lesser amounts of carbon dioxide, which is disadvantageous for the further use of the synthesis gas, for instance, for the abovementioned Fischer-Tropsch process. In other ranges of disadvantageous temperatures, less hydrogen may be formed, which, when this is desired as an alternative to the preparation of synthesis gas, may likewise be disadvantageous. In general, the reactions according to the formulae (I to III) must therefore be performed under very controlled temperature conditions in order to obtain advantageous yields and/or selectivities for the desired reaction products. This is especially true when the product should be synthesis gas.
It is therefore advantageous to control the temperature of the reaction zones in the course of the process at a level which enables rapid conversion with minimization of side reactions.
The aforementioned reactions according to the formulae (I to III) do not exhaustively represent the possible reactions in a reaction zone in which synthesis gas is to be formed according to this present invention. A very comprehensive overview over the multitude of reaction mechanisms possibly involved here is given, for instance, by A. M. De Groote and G. F. Froment in “Reactor Modeling and Simulations in Synthesis Gas Production”, published in Reviews in Chemical Engineering (1995) 11: 145-183.
The process variants disclosed here relate exclusively to reactions which are performed in fired furnaces, in which tube bundle reactors in which the reactions are performed are present. The processes are accordingly not adiabatic processes. The embodiment as a fired furnace with tube bundles is, however, required in the process according to A. M. De Groote and G. F. Froment.
In addition, A. M. De Groote and G. F. Froment disclose that this results in significant radial and axial temperature profiles in the individual reaction zones. Especially radial temperature profiles are, however, disadvantageous because there exist, as a result, in regions of the reaction zones, sites which are not operated under optimal conditions for the reaction of the hydrocarbons to give synthesis gas. Sufficient control of the temperature in the reaction zones is thus not ensured. Moreover, the reaction apparatuses disclosed by A. M. De Groote and G. F. Froment are of very complex construction, which is likewise disadvantageous since they are at least very expensive. In the event of a fault, however, the apparatus can, in particular, only be brought back into service by shutdown and repair of the overall apparatus.
Since exact temperature control is apparently impossible, there may additionally, for instance as a result of the exothermic reaction according to the formula (II), be local excess temperatures in the reaction zones, which can damage the reaction apparatus. Together with the aforementioned disadvantage of the necessarily complex construction and the associated necessary shutdown of the entire process in the event of a fault, it follows that the process disclosed by A. M. De Groote and G. F. Froment is highly disadvantageous.
EP 1 251 951 (B1) discloses an apparatus and the possibility of performing chemical reactions in the apparatus, the apparatus being characterized by a cascade of reaction zones and heat exchanger apparatuses in contact with one another, which are arranged cohesively connected to one another in an integrated system. The process to be performed here is thus characterized by the contact of the different reaction zones with a particular heat exchanger apparatus in the form of a cascade. There is no disclosure regarding the useability of the apparatus and of the process for preparing synthesis gas.
It thus remains unclear how, proceeding from the disclosure of EP 1 251 951 (B1), such a reaction is to be performed by means of the apparatus and of the process performed therein. More particularly, no process comprising endothermic reactions is disclosed.
Moreover, for reasons of unity of invention, it has to be assumed that the process disclosed in EP 1 251 951 (B1) is performed in an apparatus identical or similar to the disclosure regarding the apparatus. The result of this is that, due to the large-area contact of the heat exchange zones with the reaction zones according to the disclosure, a significant amount of heat is transferred by conduction of heat between the reaction zones and the adjacent heat exchange zones.
The disclosure regarding the oscillating temperature profile can thus only be understood such that the temperature peaks found here would be greater if this contact were not to exist. A further indication of this is the exponential rise in the temperature profiles disclosed between the individual temperature peaks. These indicate that a certain heat sink with notable but limited capacity is present in each reaction zone, which can reduce the temperature rise therein. It can never be ruled out that a certain removal of heat (for example by radiation) takes place; however, in the case of a reduction in the possible removal of heat from the reaction zone, there would be indications of a linear temperature profile or one with declining slope, since no further metered addition of reactants is intended and thus, after exothermic complete reaction, the reaction would become ever slower and the exothermicity generated would thus decrease.
Thus, EP 1 251 951 (B1) discloses multistage processes in cascades of reaction zones, from which heat is removed in an undefined amount by conduction of heat. Accordingly, the process disclosed is not adiabatic and is disadvantageous in that exact temperature control of the reaction is impossible. This is especially true of the undisclosed possibility of an endothermic reaction in the reaction zones.
An application of the process disclosed in EP 1 251 951 (B1) to the preparation of synthesis gas using the apparatuses there is disclosed by E. L. C. Seris et al. in “Scaleable, microstructured plant for steam reforming of methane” in Chemical Engineering Journal (2008) 135S:9-16.
This discloses a process using the apparatuses according to EP 1 251 951 (B1), in which synthesis gas is prepared in nine reaction zones with heat exchange zones in between. The process variant presented is declared to be multistage and adiabatic, but it is disclosed at the same time that the reaction zones are in direct contact with the heat exchange zones, as has already been disclosed in EP 1 251 951 (B1). Although this leads to an advantageous spatial integration of the reaction zones with the heat exchange zones, this at the same time has the consequence that the term “adiabatic reaction zone” is incorrect. The reaction zones are not adiabatic since they are in direct contact with the heat exchange zones at their boundaries and thus, especially given the considerable temperature gradients between the reaction zones and the heat exchange zones, a significant heat flow takes place, which is not accounted for by the convective transport of the process gases. This is disadvantageous for the purposes of exact temperature control, which is also the subject of the process presented by E. L. C. Seris et al.
Proceeding from the prior art, it would therefore be advantageous to provide a process for preparing synthesis gas, which can be performed in simple reaction apparatuses and which enables exact simple temperature control of the endothermic process, such that it allows high conversions coupled with maximum purities of the product while maintaining desired yields and/or selectivities. Such simple reaction apparatuses would be easily convertible to an industrial scale and are inexpensive and robust in all sizes.
For the endothermic catalytic gas phase reaction of hydrocarbons with steam and carbon dioxide to give synthesis gas, as just described, no suitable processes which allow this have been identified to date.
It is therefore an object of the invention to provide a process for endothermic catalytic gas phase reaction of hydrocarbons with steam and carbon dioxide to give synthesis gas, which is performable in simple reaction apparatuses with exact temperature control and which, as a result, allows high conversions coupled with high purities of the product.